This invention relates generally to gas turbine engines and more particularly to combustors used in such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. In combustors used with aircraft engines, the fuel is typically supplied to the combustor through a plurality of fuel nozzles circumferentially spaced around the forward end of the combustor. Each fuel nozzle has a surrounding assembly, known as a swirler, through which the air is supplied. Each swirler includes a number of angled vanes that impart a swirling motion to the air so as to cause the air and fuel to be thoroughly mixed.
The fuel-air mixture is ignited by an igniter mounted in the outer liner of the combustor. Under some conditions, particularly during rapid deceleration of the engine, an insufficient fuel-to-air ratio can develop and cause an engine flameout (i.e., a temporary loss of ignition). One approach to avoiding engine flameouts involves providing one of the fuel nozzles with an enriched primary circuit fuel schedule and the remaining fuel nozzles with a normal fuel schedule. The enriched fuel nozzle creates a local region of increase fuel-to-air ratio to prevent combustor flameout during rapid engine decelerations or other instances of insufficient fuel-to-air ratio. However, this approach requires that each combustor have two different fuel nozzle designs, which incrementally increases the overall production cost of the combustor.
Another known approach is to provide a plurality of identical fuel nozzles used in conjunction with an equal number of swirlers, wherein one of the swirlers is a low air flow swirler. The low air flow swirler supplies less air than the other swirlers so that it mixes the same amount of fuel as the other higher flow swirlers, but with less air. The low air flow swirler thus creates a local region of increased fuel-to-air ratio. By aligning the low air flow swirler with the igniter, the combustor is able to more easily re-ignite in the event that the remaining swirlers are extinguished temporarily during periods of insufficient fuel-to-air ratio, such as might occur during rapid engine deceleration. Because swirlers are generally easier to manufacture than fuel nozzles, it is less expensive to have two swirler designs for a single combustor than it is to have two fuel nozzle designs. Another benefit of using swirler air flow reduction instead of fuel flow enrichment to achieve an enriched fuel region is enhanced hot section durability because of lower combustor peak temperature factor at altitude conditions. That is, because less air is available at high altitude, using an enriched fuel nozzle would produce an extreme hot streak that would adversely impact durability of the hot section structure. Using air flow reduction lessens this hot streak effect.
However, it has been found that combustors using a single low air flow swirler can still experience flameouts. Accordingly, it is desirable to have a combustor design that avoids the increased production costs and lower durability of an enriched fuel nozzle and has more robust flameout prevention.
The above-mentioned need is met by the present invention, which provides a combustor having outer and inner liners defining a combustion chamber and an igniter mounted to the outer liner. A dome plate is disposed between the outer and inner liners and has a plurality of circumferentially spaced openings formed therein. A fuel-air mixer is disposed in each one of the openings; each fuel-air mixer includes a swirler mounted in the corresponding opening and a fuel nozzle received in the corresponding swirler. Two adjacent ones of the swirlers are low air flow swirlers in general circumferential alignment with the igniter and the rest of the swirlers are higher air flow swirlers. Each one of the fuel nozzles provides a similar amount of fuel so that the two fuel-air mixers having low air flow swirlers produce a local region-of increased fuel-to-air ratio in the vicinity of the igniter.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.